iWF-MAXI : Soft X-ray Transient Monitor on the ISS
N. Kawai,1 Y. Yatsu,1 N. Isobe,1 S. Sugita,1 S. Harita,1 H. Tomida,2 S. Ueno,2 K. Ebisawa,2 T. Dotani,2T. Mihara,3 M. Serino,3 M. Arimoto,4 H. Tsunemi,5 T. Sakamoto,6 T. Kohmura,7 H. Negoro,8Y. Ueda,9 T. Nakamura,9 T. Tanaka,9 Y. Tsuboi,10 M. Morii,11 M. Yoshida,12 N. Kanda,13 and M. Vagins14
1Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550
ISAS/JAXA, 3-1-1 Yoshinodai, Chuo-ku, Sagamihara, Kanagawa, 252-5210
3RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198
4Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555
5Osaka University, 1-1 Machikaneyama, Toyonaka, Osaka 560-0043
6Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258
7Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510
8Nihon University, 1-8-14 Surugadai, Chiyoda-ku, Tokyo 101-8308
9
Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502
10Chuo University, Kasuga 1-13-27, Bunkyo-ku, Tokyo 112-8551
11
Institute of Statistical Mathematics, 10-3 Midori-machi, Tachikawa, Tokyo 190-8562
12
Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima
13
Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585
14
KAVLI IPMU, 5-1-5 Kashiwanoha, Kashiwa, Chiba, 277-8583
E-mail(NK): nkawai@phys.titech.ac.jp
2

Abstract
iWF-MAXI is an X-ray transient monitor mission proposed as an experiment payload on the JEM/EF(Japanese Experiment Module Exposed Facility) of the International Space Station. Its main scientificgoals are monitor the large part of the sky in X-ray band to find and localize astrophysical transient events.
Prompt X-ray emission associated with gravitational wave events is its prime target. Other targets include
gamma-ray bursts, tidal disruption events, stellar flares, nova ignitions, supernova shock breakouts, blackhole/neutron star binaries, and active galactic nuclei (AGN). Its main scientific instrument is the Soft Xray Large Solid Angle Camera (SLC) with a localization accuracy of 0.1 degrees. Its overview and current
status are presented.
Key words: Instrumentation: all-sky monitor, X-ray transients, gamma-ray bursts, gravitationalwaves, X-ray CCD, International Space Station, WF-MAXI

1. iWF-MAXI and Overview
iWF-MAXI is an X-ray transient monitor mission
proposed as an experiment payload on the JEM/EF(Japanese Experiment Module Exposed Facility) of theInternational Space Station (ISS). It is designed to be thefirst X-ray transient monitor in the soft X-ray band (<2keV) with suﬃcient localization capability over most ofthe sky with a cadence of 90 minutes. The detector consists of combination of coded mask and CCD in X and Y
coordinates, and is placed on a replaceable small exposed
experiment platform named i-SEEP (Fig.1). iWF-MAXI
has developed with the flight proven technologies fromMAXI (Matsuoka et al. 2009) and Hitomi (Takahashi etal. 2014).
Main scientific goal of iWF-MAXI is to find and local-ize the X-ray counterparts of gravitational wave events.
The localization enable follow-up observation with X-ray
or optical telescopes, leading to measurements of theirdistance and studies on their environments and progenitors. iWF-MAXI is also expected for detection of othertransients: 1) short soft X-ray transients such as stellar flares, nova ignitions, and supernova shock breakouts, 2) short high-energy transients such as gamma-ray
bursts and tidal disruption events, and 3) Variable X-ray
sources such as black hole binaries, neutron star binaries,
and active galactic nuclei (AGN). Soft X-ray transient
search in the entire sky has huge discovery space for Xray transients.

cFig. 1. Schematic view of iWF-MAXI on ISS (⃝NASA)

2. Important Role of iWF-MAXIThe first gravitational wave event GW150916 was detected by Advanced LIGO (Abbott et al. 2016). The
waveform matched the prediction of merger of a pair ofblack holes. A lot of follow-up observation with electromagnetic telescopes were performed but the FoV ofthem were not enough compared to the localization area
by LIGO because the localization by the gravitational
wave detector alone are so large. No confirmed electromagnetic counterpart detected. In order to associate
the sources with the known astronomical objects, and/ormeasure their distances, and identify their origins, detection and localization of electromagnetic counterparts are
sorely needed. Soft X-ray band is one of the promising channel considering the huge energy density at thesource, and yet all-sky monitoring with suﬃcient sensitivity and cadence has never been performed. iWFMAXI will monitor a large area (15% of the entire sky
at any moment) with a high duty cycle (more than 50%of the total real time) to capture rare short X-ray transients. It will localize X-ray transients with positional
accuracy of 0.1 degrees. An alert with the coordinates ofthe source send to the internet for the follow-up observation within several seconds from the in-orbit detection.
With MAXI we have demonstrated the flash alert operation (Negoro et al. 2012).
3. Soft X-ray Large Solid Angle Camera (SLC)
The scientific instrument of iWF-MAXI is Soft X-rayLarge Solid Angle Camera (SLC). it consists 2 units of
CCD arrays with 1-d coded masks (X and Y directions).
We apply Hamamatsu CCD which is very similar to themodel developed for Hitomi/SXI, and with some changes
in pixel format and packaging (pin grid array instead ofwire bonding). The coded mask for localization is madeof 50 µm thick tungsten that stops X-rays with energies
below 20 keV. The mask element size is 200 µm. The
total opening fraction is about 1/3, determined basedon simulations. The energy range of SLC is 0.7 - 10 keV.
The sensitivity is 100 mCrab in 100 s (5σ) with eﬀective
area of 293 cm2 . Position localization accuracy is 0.1

degrees with FoV of 15% of the entire sky.
We fabricated the bread board model (BBM) of SLC
and performed test operations. Figure 2 shows the BBM
which consists a CCD chamber, a mechanical cooler andelectronics. One CCD was installed inside the chamber.
The set up of the SLC BBM was completed and the basic
operation of the µ-code in the drive electronics has been
confirmed. We plan to evaluate the performance of theCCDs using this SLC BBM, by the end of this fiscal year.
We also performed radiation damage test to the samemodel CCD at HIMAC. After irradiation equivalent to 8years in orbit, the SLC CCD performance was similar tothat of the spear of the Hitomi CCD (or possibly better),and the charge transfer ineﬃciency was less than 10−4 .
Dark current did not aﬀect the energy resolution anddynamic range of the CCD.

Fig. 2. SLC bread board model

References
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Takahashi, T. et al. 2014, Proceedings of the SPIE,
Volume 9144, id. 914425 24 pp. (2014)
Abbott, B.P. et al. (LIGO Scientific Collaboration andVirgo Collaboration) 2016, Phys. Rev. Lett. 116,061102
Negoro, H. et al. and MAXI Team 2012, Astronomical
Society of the Pacific Conference Series 461, 797